The present invention is related to a drive train of a wheeled vehicle having an engine and at least one driving wheel. The present invention is also related to a method for controlling the drive train of a wheeled vehicle.
A prior technology as regards a drive train of a wheeled vehicle is disclosed in a Japanese Patent Application published with a first publication No. 61-278630. The prior technology is briefly described as follows.
The drive train according to the prior technology comprises a first clutch, a transmission and a second clutch which are disposed in this order between an engine and a driving wheel. The first clutch receives driving force from the engine and transmits the driving force to the transmission. The transfer ratio of the first clutch is controlled by a driver through the operation of a clutch lever. Hereinafter, the transfer ratio designates a ratio of output torque outputted by a clutch to an input torque inputted thereto. The transmission receives driving force from the first clutch and transmits the driving force to the second clutch. The transmission ratio is controlled automatically by a control unit taking into account a throttle aperture operated by the driver and other factors, such as speed of the vehicle, etc. The transmission ratio hereinafter designates a ratio of rotational frequency of input driving motion inputted to the transmission to rotational frequency of output driving motion outputted thereby. The second clutch receives driving force from the transmission and transmits the driving force to the driving wheel. The transfer ratio is determined automatically. The second clutch is further equipped with a governor and a control unit. The governor sends a signal to the control unit, the signal being responsive to the rotational frequency of the input driving motion inputted to the transmission. The control unit automatically controls the transfer ratio of the second clutch on the basis of the output signal of the governor.
Control of the transfer ratio according to the above-mentioned construction is described more in detail as follows.
At first, while the engine speed and the speed of the vehicle are lower than respective prescribed values, that is, while the vehicle is standing still for example, the transfer ratio of the second clutch is null, that is, the driving force of the engine is disconnected from the driving wheel at the second clutch. At a same time, the transmission ratio of the transmission is kept at a high ratio. When the driver wishes to start running the vehicle, he gradually opens the throttle. Consequently, rotational frequency of the engine increases and so do the transfer ratio of the second clutch and the speed of the vehicle. At the same time, the transmission ratio decreases gradually so that the vehicle is accelerated effectively. That means, rotational frequency of the engine and the driving motion being inputted to the transmission increases more moderately than rotational frequency of the motion being inputted in the second clutch.
The construction results in the fact that the transfer ratio at the second clutch, which is controlled based on the rotational frequency of the input driving motion transmitting driving force to the transmission, increases more dully compared to an operation of the throttle.
FIG. 3(B) shows the relations of engine speed and vehicle speed for different transmission ratios while the clutches are not slipping. The two solid lines Y and Z, both passing the origin, correspond to a high transmission ratio and a low transmission ratio respectively. The value X indicates a minimum engine speed necessary for normally controlling the vehicle at a low transmission ratio. Vehicle velocity corresponding to the value X in the line Z indicates the stall speed Vo for the low transmission ratio. When the transmission ratio is low and the vehicle speed is lower than the stall speed Vo, the engine does not generate enough power necessary for controlling the vehicle. Therefore, the transmission ratio has to be increased or the clutch has to be disconnected so as to raise the engine speed. By the way, the hatched area in the figure designates combinations of engine speed and vehicle speed at which transfer ratio of the second clutch is set to be lower than 1.0 according to the prior technology. While the condition falls in the area the second clutch is slipping. Slip occurs when the engine speed is lower than the threshold value X irrespective of the speed of the vehicle. But in reality the minimum engine speed depends on the transmission ratio, and the minimum engine speed is lower than the value X when the transmission ratio is higher than the above value. Because the hatched area is applied also to higher transmission ratios, slip in the second clutch spoils a rapid acceleration of the vehicle and wastes fuel while the transmission ratio is higher.
A solution to avoid such inconveniences may be to lower the threshold to X2. But if the threshold is lowered, the engine does not generate enough power at lower transmission ratios and the acceleration is spoiled in such cases.
Further, when the vehicle is stopped rapidly, the second clutch is cut off, triggered by a sharp drop of the engine speed before the transmission ratio is sufficiently increased. Therefore, the vehicle is obliged to restart with a small transmission ratio resulting in a dull acceleration. It is because the transmission ratio is changeable only when the transmission is rotating in the case of an automatic transmission having an endless belt for transmitting torque.